Precipitation or depositing of particles from a solution

ABSTRACT

The invention relates to methods of controlling the precipitation or depositing of particles from a solution in a cell in which an electric field is applied between an anode and a cathode, in particular the precipitation of metal compounds such as oxides and hydroxides and the coprecipitation of mixed metal oxides from a solution of the metal salt(s) in an electrolysis cell, but also the electrolytic deposition of cathodic metal deposits as well as the electrophoretic deposition of colloidal particles.

BACKGROUND ART

Methods of electrolytically co-precipitating mixed metal oxides are known from Canadian Patent No. 593,187 and UK Patent No. 864.249. Another method of precipitating or co-precipitating metal compounds, more particularly oxides and hydroxides, in an electrolysis cell is described in Canadian Patent No. 623.339.

It was recognized in the latter patent that the process should be carried out at a specific and as constant as possible pH, and this was achieved by carrying out a preliminary electrolysis to obtain suitable starting conditions and setting the electrolysis currennt at a value corresponding to the rate of supply of a solution of the salts from which the compounds are precipitated. In carrying out these known processes, it has been observed that the particle size and density do not remain constant and this reduces the usefulness of the product. The invention is based on the observation that the uneven particle size and variable density obtained with the known processes are due to variations in the solution pH which occur when following the prior teachings and because of the inherent difficulties in measuring and controlling the pH in an electrolysis cell particularly when gas is bubbled in or is electrochemically generated.

Similar considerations apply to the control of the particle size and density of electrodeposited coatings and to the control of electrophoretic deposition.

DISCLOSURE OF THE INVENTION

According to the invention, the precipitation or the depositing of the particles is controlled by measuring the pH of the solution in the cell using a probe shielded from migrating electric current and adjusting the pH of the solution to a selected value as a function of the measured pH.

Preferably, this is achieved by supplying a first electrical signal representative of the measured pH, comparing the first electrical signal with a reference electrical signal corresponding to the selected pH, and adjusting the pH of the solution as a function of the difference of the first signal and the reference signal to maintain it close to the selected pH.

The pH may be adjusted by acting on any pH-determining parameter such as by bubbling acid vapour or base vapour into the solution or by adjusting the supply of air, in the case where air is supplied as an oxidant. This method of bubbling in a vapour or air is particularly appropriate when a solution of metal salt(s) to be precipitated is obtained by dissolving at least one metal anode.

In another arrangement, the pH is adjusted by varying the electrolysis current. For example a preprepared solution of metal salt(s) is introduced into a compartment of the electrolysis cell in which the particles are precipitated, and the ions liberated by the salt(s) are passed through a separator into another compartment of the cell by passing an electrolysis current at a raate controlled as a function of the measured pH to keep the pH of the solution at the selected value. This arrangement is recommended when it is desired to incorporate a metal such as barium which does not dissolve anodically or manganese which is soluble without the passage of current so that, when connected as an anode, it does not dissolve in proportion to the current passed. However, the arrangement can of course also be used to supply the salts of other metals.

The solution of salts can be introduced into the cell compartment by dripping it in uniformly or by blowing in a fine dispersion of the solution for example with an oxidizing gas, or in any other suitable manner.

It is possible to combine this arrangement with the previous one by providing ions of at least one further metal to be precipitated by dissolving at least one metal anode, and adjusting the pH of the solution by bubbling acid vapour, base vapour or by adjusting the supply of air into the solution when said controlled electrolysis current reaches a threshold value.

It is equally possible, when a preprepared solution of the metal salt(s) is supplied to the electrolysis cell, and whether or not this is combined with dissolving at least one metal anode, to supply a substantially constant electrolysis current calculated to correspond more or less exactly to the rate of supply of metal salt to the solution so that ions liberated by the salt(s) are passed through a separator into another compartment of the cell at a corresponding rate, and to adjust the pH to the selected value by bubbling in acid vapour, base vapour or by adjusting the supply of air as a function of the measured pH.

Advantageously, to avoid disturbances of the pH measurement in the electrolyte solution, the pH is measured by a probe disposed in a tube which shields the probe from gas bubbles and stray currents carried by migrating ions. This may be a tube which extends down to below a source of gas bubbles, or the end of the tube may be closed with an electrolyte-permeable but gas-bubble impermeable gauze. In some instances, instead of a tube, a U-shaped section or even two parallel spaced plates may be sufficient to shield the probe from the influence of stray current carried by migrating ions.

In most cases, an oxidizing gas is supplied to the electrolyte solution to precipitate an oxide or mixed oxide of the metal or metals. It is also possible to electrochemically generate an oxidizing agent in situ in the solution using an insoluble anode situated in the precipitating zone, possibly operating in conjunction with soluble enodes and preferably with an electrolyte such as sodium sulphate suitable for the generation of oxygen. Alternatively, hypochlorite can be generated as the oxidizing agent by using a sodium chloride or potassium chloride electrolyte. When an auxiliary anode is used to generate an oxidizing agent in situ, it is possible to control the pH of the electrolyte by adjusting the current supplied to the auxiliary anode as a function of the measured pH.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of an electrolysis cell and associated equipment;

FIG. 2 is a circuit diagram of a control unit; and

FIGS. 3 and 4 are schematic representations of further arrangements.

BEST MODE FOR CARRYING OUT THE INVENTION

As shown in FIG. 1; an electrolysis cell comprises a housing 1 containing an electrolyte 2 in which a cathode 3 and anodes 4 and 5 dip. The cathode 3 is connected to the negative terminal of two D.C. power sources 6 and 7 whose positive terminals are connected to respective anodes 4 and 5 so that these anodes can be supplied at different voltage and hence different current density. The cathode 3 may, for example, be of iron and the anodes 4, 5 of iron and nickel respectively, andd the electrolyte may be an aqueous alkali metal salt solution such as KCl in which the anode metals dissolve proportionately to the respective currents that they pass. It is understood that two anodes are shown by way of example; any convenient number of anodes of different metals could be used, each supplied with current to dissolve at a desired rate, or a single anode consisting of a metal (e.g. iron) or an alloy (e.g. iron/nickel) could be used.

Under the anodes 4, 5 extends an air tube 8 through which air is delivered from an air pump 9, this air serving to oxidize the dissolved metals which are co-precipitated in the electrolyte as a mixed oxide powder, e.g. a nickel ferrite powder when iron and nickel anodes are used. Other oxidizing mediums could be used, such as oxygen, ozone, chlorine, hypochlorite, hydrogen peroxide and so on. It is also possible to generate an oxidizing agent by electrolysis.

As shown, preferably the air pump 9 is connected to the tube 8 via a jar 22 advantageously containing filter material 25, for example glass beads, a second jar 24 containing 25% KOH 25 and a flow meter 28 which is set to control the rate of supply of air by the pump 9 according to the quantity of particles to be kept in suspension in the oxidizing zone of the electrolyte 2. The KOH in jar 25 removes carbon dioxide from the air and prevents the unwanted formation of ferrous carbonate in the housing 1.

The purpose of the filter material 23 is to prevent fumes from reaching the air pump 9; however, in installations with a large jar 22 the filter material 23 may be omitted. Also, if desired another similar filter jar may be connected between the jar 24 and tube 8.

The tube 8 may simply be an open-ended tube but it is preferable in larger installations to use a distributor 27 perforated in its upper face with perforations of a selected size which produce relatively fine or large bubbles depending on the quality and dimensions of the powder being produced.

In the bottom of the housing 1 is an optional magnetic stirrer 26, it being understood that in instances where very magnetic powders are being produced, mechanical stirring means will be preferred.

The pH of the electrolyte 2 is measured by a glass probe 10 which is inserted in the electrolyte 2 and is sheltered from the migrating electric current between the electrodes 3, 4 and from rising gas bubbles by being enclosed in a tube 10A having at its lower end a gauze 10B of suitable nonmetallic material such as PTFE or PVC which is permeable to the electrolyte but not to the gas bubbles. In this manner, the probe 10 accurately measures the pH of electrolyte ascending by convection in the tube without disturbance from air bubbles or stray current carried by migrating ions. This probe 10 is connected to a pH meter, which provide a visual display of the measured pH and an analog electric output signal 12 corresponding to the measured pH. The signal 12 is delivered to a control circuit 13, described in detail later, which compares the signal 12 with a reference signal corresponding to a selected pH, and switches on or off an air pump 14.

When actuated, the air pump 14 blows air through a tube 15 which extends into a jar 16 containing filter material 17, for example glass beads, into which the tube 15 dips. This filter material 17 prevents acid fumes from penetrating into the air pump 14. The top part of filter jar 16 is connected by a tube 18 to another jar 19 containing concentrated hydrochloric acid 20, into which the tube 18 dips. Finally, a tube 21 extends from the space in jar 19 above the hydrochloric acid 20 to the electrolyte 2 in vessel 1, the tube 21 terminating just below the space between cathode 3 and anodes 4, 5. If desired, this tube 21 may be fitted with a perforated distributor like distributor 27.

Thus, when the air pump 14 is switched on, the air blown along tubes 15 and 18 drives air containing acid vapour along the tube 21 and this acidified air is delivered into the electrolyte 2 and bubbles up through the zone where the co-precipitation of the metal oxides is taking place. The air delivered by tube 21 thus acts in the same way as the air delivered by tube 8 to oxidize the metal ions. The minute particles of acid delivered are very evenly distributed by the bubbling air in the co-precipitation zone of the electrolyte 2 and/or on the co-precipitated particles, and this ensures a very homogeneous distribution of the acid in the co-precipitation zone. This supply of acidified air continues until the pH has dropped to the pre-selected value, corresponding to a desired particle size.

With reference to FIG. 2, the pH control circuit 13 is connected to the A.C. mains by a transformer 30 which provides, by means of diodes D1 and D2 and capacitors C1 and C2, a stable D.C. input powering an operational amplifier 31. The output signal 12 of pH meter 11 is connected to a resistance bridge formed by resistors R1 and R2 and to one input, 32, of the operational amplifier 31. The input signal on 32 is thus a voltage which fluctuates in proportion to the measured pH. The other input 33 of amplifier 31 is connected to a potentiostat P1 which provides an input signal on 33 of constant voltage, but which can be set at any value corresponding to a selected pH according to the desired particle size and density.

It can readily be seen that by setting the potentiostat P1 appropriately in relation to the values of resistors R1 and R2, the signal on 33 can, if desired, be set equal to the signal on 32 and in fact this is what is usually done at the beginning of operation: the pH of the electrolyte 2 is brought to a selected value according to the desired particle size, and the potentiostat P1 is set to a corresponding pH value.

Via a resistor R3, a transistor 34 and a relay 35 is stabilized by a capacitor C3, operational amplifier 31 controls a switch 36 controlling the air pump 14. As shown, the switch 36 controls two circles for actuating the pump: one circuit for use when the pump 14 actuates the supply of acid or air, the other circuit for use when the pump 14 actuates the supply of a base. In the instance described with reference to FIG. 1 where there is acid 20 in the jar 19, the circuit corresponding to the supply of acid is used. In this case, as long as the signal on 32 does not exceed the reference signal on 33, the amplifier output is zero, switch 36 remains open and the air pump 14 is off. However, when the pH of the electrolyte 2 rises so that the signal on 32 exceeds the reference signal on 33, this triggers the amplifier 31 and the switch 36 is closed (as shown) by energisation of the coil of relay 35. As soon as the pH of the electrolyte 2 is brought back to the reference value by the supply of acidic air, the amplifier drops to zero and the switch 36 springs open.

With the described pH measuring and control system, the pH can be controlled very accurately, to about +/-0.05, and because the pH of the precipitation zone can be maintained homogeneous at a chosen pH, particles of a specified uniform size and density can be obtained.

FIG. 3 shows an arrangement in which a solution of salts for conversion to a mixed oxide into an electrolysis cell from a burette 40. The cell has a housing 41 containing an electrolyte 42 such as KCl or NaCl. The cell housing 41 is placed on a heater 43. Cell housing 41 is divided by a diaphragm or membrane 44 into an anode compartment containing an anode 45 and a cathode compartment containing a cathode 46.

The anode 45 is preferably a dimensionally-stable anode consisting for example of a sheet of expanded titanium mesh coated with an electrocatalytic coating such as a so-called "mixed crystal" of ruthenium oxide-titanium oxide. The cathode 46 may consist of iron, nickel or stainless steel. The diaphragm or membrane 44 is off the anion-exchange type, i.e. in the case where a mixture of chlorides is supplied by the burette 40 it will allow chloride ions to migrate from the cathode compartment into the anode compartment.

Into the cathode compartment extends a tube 47 through which air can be supplied in the vicinity of cathode 46.

The arrangement so far described corresponds substantially to the prior-art arrangement of Canadian Patent No. 623,339. According to the practice of the prior art, current was passed between the anode 45 and cathode 46 to bring the pH in the anode and cathode compartments to desired values. Then a solution of metals to be precipitated, for example, a mixture of FeCl₂, NiCl₂ and MnCl₂ in a desired ratio, is introduced into the cathode compartment from the burette 40, and the current and the rate of supply of the solution were set so that the concentration of precipitating ions in the cathode compartment remained substantially constant. Thus chloride ions from the supplied chloride solution migrate through the diaphragm of membrane 44 and are released as chlorine gas at the anode 42. In the cathode compartment, the dissociated iron, nickel and manganese ions are oxidized by the air supplied through tube 47 and are precipitated as a mixed oxide. The starting-up of this process is complicated, and any fluctuations in the current supplied or the rate of supply of the solution can upset the equilibrium of the process so that very careful monitoring is required.

According to the invention, the arrangement is provided with a pH probe 48 dipping into the cathode compartment and shielded in a tube 48A with an electrolyte-permeable but gas-impermeable PTFE or PVC gauze 48B. This tube protects the probe from the effect of stray current carried by migrating ions and from the ascending gas bubbles, so that the pH measurement is not disturbed. The probe 48 supplies an analog signal proportional to the measured pH to a pH control circuit 49 which compares this signal with a pre-set control value corresponding to a desired pH. The circuit 49 in turn supplies an analog output signal to a DC source 50. This DC source is connected to the anode 45 and cathode 46 so that the electrolysis current supplied is exactly that which required to maintain a constant pH in the cathode compartment, whatever may be the fluctuations in the rate of supply of the mixed chloride solution from burette 40. Startup of the co-precipitation process is thus greatly facilitated and optimum conditions for co-precipitation at an exactly set pH can be maintained easily.

FIG. 4 shows a hydrid arrangement which producs mixed oxides by the combined action of dissolving one or more soluble anodes (as in FIG. 1) and supplying a solution of at least one metal salt (as in FIG. 3). The cell of FIG. 4 has a housing 61 containing an electrolyte 62 such as KCl or NaCl, the housing 61 being placed on a heater 63. The cell housing 61 is divided by an anion-exchange diaphragm or membrane 64 into a first compartment containing a dimensionally-stable anode 65 and a second compartment containing a cathode 66, for example of iron, and two auxiliary soluble anodes 67 and 68, for example of nickel and iron. Into this second compartment extends a tube 69 through which air or an air/ammonia mixture is supplied and bubbled under the cathode 66 and anodes 67, 68 through a perforated distributor 70. Above the second compartment is a burette 71 from which a solution of one or more salts, for example a solution of manganese chloride, can be supplied adjacent to the cathode 66. In the zone of electrolyte 62 between or adjacent to the cathode 64 and soluble anodes 67, 68 dips a glass pH probe 72 enclosed in an open-ended glass tube 73 which extends to below the distributor 70. This tube 73 protects the pH probe 72 from the effect of stray current carried by ions migrating between the cathode 66 and anodes 67, 68 and from the effect of ascending gas bubbles. The probe 72 thus accurately measures the pH of electrolyte ascending by convection in the tube 73.

The probe 72 supplies an analog signal proportional to the measured pH to a control circuit 74 which compares this signal with a pre-set control value corresponding to a desired pH. The circuit 74 in turn supplies an analog output signal to a DC source 75 connected to the anode 65 and cathode 66 so that the electrolysis current supplied is exactly that which is required to maintain a constant pH in the second compartment.

As in the arrangement of FIG. 1, the anodes 67, 68 are connected to separate DC supply sources with a common cathodic connection with source 75 and cathode 66, so that the anodes 67, 68 can be supplied at different current densities and dissolve proportionately to the different currents that they pass. The rate of supply of MnCl₂ solution from burette 71 is correlated to the current supplied to anodes 67, 68 and hence their rates of dissolution so that a mixed oxide (manganese-nickel ferrite) of desired composition is obtained.

In normal operation, carbon dioxide-free air is supplied via tube 69 to ensure oxidation of the dissolved metals. However, as a modification of the process, it is also possible to intermittently supply an air/ammonia mixture as an additional means of keeping the pH in the second compartment constant. This can be achieved using the circuitry of FIG. 2 with the switch 36 controlling the circuit for the supply of a base (ammonia). Or the pH control circuit could be arranged so that when the analog signal of circuit 74 (and hence the current between anode 65 and cathode 66) reaches a threshold value, the supply of air/ammonia is actuated.

EXAMPLE I

The arrangement of FIGS. 1 and 2 was used for the production of a nickel ferrite using nickel and iron anodes, the anode currents being arranged to dissolve the metals to give a theoretical composition of equimolar Fe₂ O₃ /NiO. The electrolyte was 5% KCl at 70° C. The electrolysis was carried out both with control of the pH between 6.2 and 6.4 by the intermittent supply of acid air using the described pH measurement and control device and, for comparison, using the prior art method without servo-control of the pH in the cell. The powders obtained were dried and examined by transmission electron microscopy. The measured particle size ranges of greater than 50% of the particles (PSR 50) and the total particle size ranges (TPSR) are shown in Table I.

                  TABLE I                                                          ______________________________________                                                        PSR 50 (μ)                                                                          TPSR (μ)                                             ______________________________________                                         controlled particles  0.009-0.02                                                                               0.005-0.08                                     pH         flakes              0.02-0.13                                       prior art  particles  0.12-0.24                                                                               0.03-0.40                                                  flakes              0.04-0.80                                       ______________________________________                                    

It can readily be seen from this table that the particles/flakes produced are considerably finer and more uniform when the pH is controlled according to the invention.

It was further observed that when the pH control device was used, the intermittent supply of HCl vapour continued throughout the process with variable intervals.

Also, the pH range of 6.2-6.4 was chosen as this is found to give the most dense nickel ferrite. When a less dense product is required, the pH is set at a selected higher value.

EXAMPLE II

The arrangement of FIGS. 1 and 2 was used for the production of iron oxide (ferrite) using a single soluble iron anode in a 5% KCl electrolyte. The pH was controlled at 6.2-6.4. The process parameters were varied by using the electrolyte at 70° C. or 90° C. and by blowing in very fine air bubbles or larger air bubbles, or with in situ oxidation using an auxiliary dimensionally stable electrode.

In all instances, it was found that the pH control circuit operated to intermittently supply HCl vapour at varying intervals during a start-up phase of about 30 minutes to 1 hour and thereafter the process reached a steady state at constant pH with little or no further additions of HCl vapour.

Also, in all instances, a fine uniform powder was obtained. Typical size ranges measured by transmission electron microscopy are: PSR 50 : 0.03-0.1 μ, TPSR: 0.03-0.5 μ. Furthermore, the particles were analysed for their contamination with potassium. The average potassium content was 45 ppm with a maximum of 59 ppm and a minimum of >5 ppm.

When it was attempted to carry out the same process with the pH control device switched off, it was found that the pH progressively rose from 6 to about 9 and instead of fine particles a sludge was produced. Analysis of the product showed a potassium contamination of the order of 1900-2000 ppm.

Also, when the process was carried out at about 75° C. or below, it was found that the pH control could be achieved by blowing in an extra controlled amount of air (freed from CO₂ by passing through KOH) without acid vapour. At higher temperatures the acid vapour is needed to compensate for evaporation of acid fumes from the cell.

INDUSTRIAL APPLICABILITY

The particies obtained by precipitating at controlled pH according to the invention will often be oxides which, on account of their uniformity and extremely fine size, are useful in many applications especially as permanent magnets and in electronics applications for ferrite-based materials.

One particular application for the fine and uniform particles obtained according to the invention is in the manufacture of electrodes for electrolytic processes, such as the ferrite-based electrodes for molten salt electrolysis disclosed in PCT publication WO 81/01717. In manufacturing such electrodes, the particles obtained according to the invention are usually cold pressed isotatically then sintered at an elevated temperature (about 1350° C.) under argon or with a low oxygen pressure. The uniformity and fineness of the particles considerably simplifies the sintering operation and gives very high density electrode bodies (more than 90% of the theoretical density). The particles can be sintered alone or in admixture with particles of other materials. Electrodes produced using the fine and uniform powders obtained according to the invention have enhanced stability because of the homogeneity of the particles.

Another application of the invention is in the production of fine metal powders by precipitating particles of the oxides (or hydroxides) and subjecting them to reduction to give a very fine and homogenous powder of the metal(s). This is for instance useful in producing very fine iron particles which would be pressed into a body for use as an electrode in batteries such as the iron-air or iron-nickel battery.

It will be appreciated that the described embodiments of pH control for the precipitation of particles in an electrolysis cell apply also to electrodeposition cells particularly where it is desired to obtain cathodic deposits of desired uniform grain size, crystal structure and porosity all of which may be adversely affected by small fluctuations in the pH adjacent the depositing zone. Using the described pH control devices, where the pH is measured in or very close to the depositing zone, uniform cathodic deposits can be grown to the desired thickness with great simplification of the process control. Likewise, the electrophoretic deposition of charged colloidal particles is pH dependent and small fluctuations of the pH affect the rate of deposition and the deposit quality and uniformity. By controlling the deposition as a function of the pH measured in or close to the depositing zone according to the invention, the deposit quality can be improved and the process control simplified. 

We claim:
 1. In a method of precipitating or depositing particles from a solution in a cell in which an electric field is applied between an anode and a cathode, and wherein there are gas bubbles in the solution, the improvement comprising controlling the precipitating or depositing by measuring the inside of the cell the pH of the solution in the cell using a probe situated inside of the cell, said probe being shielded from migrating electric current and from gas bubbles, and by adjusting the pH of the solution to a selected value as a function of the measured pH.
 2. The method of claim 1, characterized by supplying a first electrical signal representative of the measured pH, comparing the first electrical signal with a reference electrical signal corresponding to the selected pH, and adjusting the pH of the solution as a function of the difference of the first signal and the reference signal to maintain it close to the selected pH.
 3. The method of claim 1, wherein the pH of the solution is adjusted by bubbling acid vapour, air or base vapour into the solution as a function of the measured pH.
 4. The method of claim 1, 2, or 3, in which metal compounds are precipitated from a solution of metal salts obtained by dissolving at least one metal anode.
 5. The methhod of claim 1 or 2, in which metal compounds are precipitated froma preprepared solution of metal salts introduced intoa compartment of the cell in which the particles are precipitated, and ions liberated by the salts are passed through a separator into another compartment of the cell by passing an electrolysis current at a rate controlled as a function of the measured pH to keep the pH of the solution at the selected value.
 6. The method of claim 5, wherein ions of at least one further metal to be precipitated are obtained at least one metal anode, and the pH of the solution is adjusted by bubbling acid vapour, air or base vapour into the solution when said controlled electrolysis current reaches a threshold value.
 7. The method of claim 1, 2, or 3, wherein the pH measurement probe is displaced inside of a tube which shields the probe from the migrating current and from the gas bubbles.
 8. The method of claim 7, wherein an oxidizing gas is supplied to the solution to precipitate oxide of metal salts dissolved in the solution.
 9. The method of claim 1 or 2, wherein an oxidizing agent for precipitation of oxide of metal of salts dissolved in the solution is generated in situ in the solution by an insoluble anode.
 10. The method of claim 9, wherein the current supplied to said insoluble anode is controlled as a function of the measured pH.
 11. In a method of precipitating particles from a solution in a cell in which an electric field is applied between an anode and a cathode, and wherein oxidizing gas is supplied to the solution to precipitate oxide(s) of metal salts dissolved in the solution, the improvement comprising controlling the pH and thereby controlling the precipitating of the particles by measuring inside of the cell the pH of the solution in the cell using a probe situated inside of the cell, said probe being disposed inside of a tube which shields the probe from migrating electric current and from gas bubbles, and by adjusting the pH of the solution to a selected value by bubbling acid vapour, air or base vapour into the solution as a function of the measured pH.
 12. The method of claim 11, wherein the oxidizing gas is supplied continuously and the acid vapour, air or base vapour is supplied intermittently as a function of the measured pH.
 13. In a method of precipitating particles from a solution in a cell in which an electric field is applied between an anode and a cathode, and wherein a prepared solution of metal salts is introduced into a first compartment of the cell and oxidizing gas is supplied to the solution to precipitate oxide(s) of the metal salts, and ions liberated by the salts are passed through a separator into another compartment of the cell by passing electrolysis current, the improvement comprising controlling the precipitating of the particles by measuring inside of said first compartment of the cell the pH of the solution in said first compartment using a probe situated therein, said probe being disposed inside of a tube which shields said probe from migrating electric current and from gas bubbles, and by adjusting the pH of the solution to a selected value by passing said electrolysis current as a function of the measured pH to keep the pH of the solution at the selected value. 